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Which of the following is stated in the passage?

Good work!

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The passage states that the Doppler Effect shows scientists that the universe is constantly expanding, which means that the objects within it are moving away from each other.

It also states that the Doppler Effect is a change in the perceived output of a source of waves (i.e. that the actual frequency of the emitted waves does not change, as stated in the first sentence) as a result of movement between the source of waves and the observer (i.e. that either one or both can be in motion).

Incorrect.

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This answer choice incorrectly represents the definition of the Doppler Effect, as it says that the source of waves and the observer must be in motion. However, the passage tells us that a source of waves must be in motion with respect to an observer, i.e. that either the source, the observer or both may be in motion.

Incorrect.

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This answer choice is eliminated by the questionable claim that objects outside our galaxy are moving towards us. From the last sentence, we know that the Doppler Effect shows a constant expansion of the universe. If the universe itself is expanding, the objects within it must be moving away, not towards, us.

While some objects may be moving towards us, depending on our relevant locations in an expanding universe, this answer choice is partial at best.

Incorrect.

[[snippet]]

This answer choice is eliminated by the questionable claim that the objects in the universe are constantly expanding. The Doppler Effect shows scientists that the universe itself is constantly expanding. We know nothing about whether the objects within it are themselves expanding as well.

Incorrect.

[[snippet]]

This answer choice incorrectly tells us that the Doppler Effect occurs when an objects which emits waves changes the frequency of those waves. However, the first sentence in the passage states that the source of waves emits them at a constant rate and that, due to the relative motion between the source and the observer, the latter perceives those waves at a different frequency.

The Doppler Effect, which tells scientists that the universe is constantly expanding, is marked by a changed perception of the waves emanating from a moving source of waves and a moving observer.
The Doppler Effect, which tells scientists that objects in the universe are steadily moving farther from each other, results from movement between a source of waves and the observer that alters the source's perceived frequency.
The Doppler Effect, which tells scientists that objects outside our galaxy are moving towards us, is marked by a changed perception of waves when either the source of the waves, the observer or both are moving.
The Doppler Effect, which tells scientists that objects in the universe are constantly expanding, is marked by a change in perceived frequency of waves when the source of the waves and the observer are moving relative to each other.
The Doppler Effect, which tells scientists that the universe is constantly expanding, results from a change in emitted frequency when a source of waves and an observer are moving relative to each other.